The present invention relates to gas turbine engines. More particularly, the present invention relates to vibration damping for a bearing support assembly for a gas turbine engine.
A gas turbine engine includes rotating spools or rotor shafts with blades that compress air needed for operation. One of the major goals in gas turbine engine fabrication is to optimize efficiency of the compressor and the turbine so that work is not lost. Tip leakage between the blades and the surrounding case is a significant source of engine inefficiency.
At shut-down of the gas turbine engine, the engine may cool unevenly. For example, the bottom part of the engine usually cools more quickly than the top portion of the engine as the heat within the engine migrates radially outward. This temperature differential in the gas turbine engine can cause the rotor shafts to distort and become eccentric relative to an engine center axis, taking on an effectively arcuate shape instead of being generally cylindrical. This phenomenon is also known as the “bowed rotor effect.” A “bowed” rotor shaft is problematic at engine start-up because the distortions in the rotor shaft may result in the blades rubbing into the surrounding case or rub strip thereby creating larger gaps between the blade tips and the surrounding case and increasing tip leakage and engine inefficiency. In some cases, the eccentric movement of the bowed rotor shaft can also create gaps between many of the components in the compressor and/or turbine sections of the gas turbine engine that can also cause air leakage and decrease engine efficiency. In addition to increasing tip leakage and engine inefficiency, a bowed rotor can also create noise and vibrations felt in the airplane at start-up.
Squeeze film dampers can be used in a bearing support assembly that supports a rotatable shaft of a gas turbine engine for vibration reduction. The bearing is contained within a stationary housing. A bearing centering spring, also known as a squirrel cage, supports the bearing. The squirrel cage is connected to the stationary housing such that the squirrel cage does not rotate relative to the stationary housing, yet the squirrel cage is flexible enough to bend in response to loads experienced from the bearing. A film of liquid, such as oil, is supplied to an annulus disposed between the stationary housing and the squirrel cage. The film of oil reduces or damps engine vibration and the transmission of vibrations from the bearing to the engine structure.
When the compressor or turbine shaft rotates, imbalance or rotor instabilities can cause excessive whirling of the shaft and vibration of the engine, engine mounts and airframe. As a speed of the shaft increases, the rotating shaft passes through a variety of critical speeds, which occur when the speed of the rotating shaft traverses a resonant frequency of one of the various structures of the engine. Due to the multitude of structures that make up any given engine, the rotating shaft passes through several critical speeds as the rotating shaft accelerates. As the rotating shaft rotates, the rotating shaft may be displaced from a centerline of the engine. The rotating shaft then whirls about a centerline of the engine. As the bearing moves with the rotation of the shaft, the squirrel cage vibrates within the stationary housing and squeezes the oil in the annulus.
Typical fluid film bearing dampers have a fixed geometry, which causes inefficiencies with regards to bearing stability and transmission of vibrations due to a set film depth only accommodating a single operating condition. These existing bearing dampers are limited to occupying an on or off state based on being supplied with oil.